1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to a system for imaging of recording media such as lithographic printing members.
2. Description of the Related Art
Imaging devices that utilize laser power sources require delivery of the laser output to a working surface of the recording medium. It is important, when focusing radiation onto the recording blank, to maintain satisfactory depth-of-focus--that is, a tolerable deviation from perfect focus on the recording surface. Adequate depth-of-focus is important to construction and use of the imaging apparatus; the smaller the working depth-of-focus, the greater will be the need for fine mechanical adjustments and vulnerability to performance degradation due to the alignment shifts that can accompany normal use. Depth-of-focus depends on numerous factors, including the characteristics of the laser itself, its output beam divergence, and the optical arrangement used to transport the laser output and focus it.
An ideal laser emits "single-mode" radiation--that is, a beam having a radially symmetric Gaussian energy distribution.
The bulk of the beam's energy is concentrated in a single, central peak, and falls off radially and smoothly in all directions according to the Gaussian function. Single-mode radiation not only enhances depth-of-focus, but also produces clean image dots with crisp, circular outline contours.
Unfortunately, not all recording constructions are imaged at wavelengths for which single-mode lasers are available. Instead, the imaging lasers produce beam profiles having uneven intensities. The beams are "multi-mode," exhibiting several (or numerous) intensity peaks rather than a single dominant peak. The dots they produce on a recording construction have multiple "hot spots" rather than a single, central region of maximum imaging intensity.
In graphic-arts applications, such as imaging of lithographic printing plates, these uneven image dots can prove highly disadvantageous. The cumulative effect of ragged image dots is a general degradation of image quality. Moreover, the sharp multi-mode peaks reduce depth-of-focus, since deviation from ideal focus causes their energy flux densities to fall off far more rapidly than would be the case with single-mode peaks.